In the diagnosis of osteoporosis, it is useful to measure the ratio of inorganic mineral density to organic matrix density. This quantity, which is referred to as the “degree of bone mineralization,” is now measured invasively. In histomorphometry, thin sections of bone are removed from a patient and examined under a microscope. Alternatively, samples of bone are subjected to destructive chemical analysis or biopsies.
Because of their difficulty, invasive methods such as these are undesirable for screening large numbers of people for osteoporosis. In addition, these methods require inferring the degree of bone mineralization using a limited number of samples.
Magnetic resonance imaging techniques are useful for imaging internal structures. Such techniques typically include placing a patient in a non-uniform magnetic field and applying an RF (radio-frequency) pulse. After the RF pulse is complete, one waits for short time before collecting data. The length of this wait, which is referred to as the “dead time,” depends on how quickly the RF pulse decays to the point where data collection can safely begin.
After the dead time has elapsed, one can then collect data for a limited time, which is determined by the spin—spin relaxation time (often referred to as “T2” or “T2*”). The interval between the end of the dead-time and the end of the spin—spin relaxation time defines a data collection window.
When applied to solid, relatively dry structures, such as bone, MRI is hampered by extremely short spin—spin relaxation times. With the dead time now being an appreciable fraction of the spin—spin relaxation time, the data collection window becomes so short that the amount of data collected is insufficient for forming a useful image.